How to gain weight fast, must refer this useful and healthy tips without using any suppliements or other weightgain products.

How to gain weight instant:
Increase protein intake
Never let your protein intake go lower than 1g/lb of body weight when you are aiming to add long-term muscle. It's the building block of muscle, and your body is going to be starving for it.

A. Foods that help you gain weight
Eat these foods which help you gain weight.
Whole eggs: It is economical and loaded with protein, vitamins A,D, E and good cholesterol.
Butter: Butter is linked with long term weight gain. Intake of butter should be in moderation, as regular consumption of butter is bad for your heart.
Tuna: The vital fatty acids in tuna contain a host of healthy fats, which not only aid in weight gain but also help in maintaining physical well-being.
Natural fruit juice: A healthy and nutritious way to gain some pounds, sip 100-per cent fruit juice that's full of sugar and added nutrients.
Whole wheat bread: You can eat healthy and pack on pounds at the same time by eating whole wheat bread which accounts for approximately 69 calories per slice.

B. Food rich in good calories
Peanut butter - Spread - soft, salty peanut butter on bread. It gives you approximately 192 calories plus it is extra high in protein.
Energy bars - It is loaded with good-for-you nuts and oats. One bowl packs around 500 calories, which is healthy as well as yummy for breakfast.
Cheese - One serving of cheddar cheese packs a good 69 calories. Cheese is concentrated milk, so it is high in protein and calcium, fat, cholesterol and cheese calories.
Oil - Adding extra fat to your food is an easy way to add calories, choose olive oil, canola oil, etc. which is good for your health and will add those much needed calories too.